1. Field of the Invention
The present invention relates to a frictional transmission which is assembled in a power transmitting unit of mechanical apparatus of various kinds for decelerating a rotational motion and increasing a torque from, for example, an electric motor, or increasing a rotating speed to transmit the same to an output unit.
The present invention also relates to an electric power steering apparatus which is assembled in a steering apparatus of an automobile for reducing a power required for the driver to operate a steering wheel by utilizing the electric motor as an auxiliary power source.
2. Related Background Art
A transmission for varying and transmitting a rotating speed is employed in mechanical apparatus of many kinds. As such a transmission, there is conventionally used a frictional speed decreasing apparatus, in addition to a geared transmission including a worm transmission, planetary reduction gear, etc.
Out of these devices, in the geared transmission which is used generally, it is required to have a double-line arrangement of gears or to use a large-sized gear, for obtaining a large ratio of speed change (a ratio of speed decrease or a ratio of speed increase). For this reason, it is difficult to realize a geared transmission of a small size and capable of obtaining a large ratio of speed change. Also, since it is required to set a backlash in the geared transmission, a shake is inevitably generated to some extent in a transmission part thereof, which may bring about a problem of the transmission in some use.
Also, in the case of a worm transmission, it is possible to obtain a large ratio of speed decrease in a comparative small size. However, it is impossible to obtain a large ratio of speed increase if the worm transmission is of small size. Moreover, in the case of the worm transmission, not only a shake is inevitably brought about, like in the case of an ordinary geared transmission, but also a high efficiency can be hardly obtained.
On the other hand, in the case of a frictional transmission, there is no need to set a backlash, unlike in the case of the geared transmission, so that no shake is brought about. As such a frictional transmission, there is conventionally known that disclosed in Japanese Patent Application Laid-Open No. 6-288453. The frictional transmission disclosed in this Laid-Open gazette is arranged by bringing the outer peripheral surface of a small-diameter taper roller having a tapered form and concentric with an input shaft and the outer peripheral surface of a large-diameter taper roller having a tapered form concentric with an output shaft into frictional engagement.
However, in the case of the frictional transmission disclosed in this Laid-Open gazette, a contact pressure between the outer peripheral surfaces of the two taper rollers cannot be stabilized from the first stage of the power transmission. Consequently, the efficiency of power transmission is deteriorated at this first stage.
As an apparatus for reducing the power which is required for the driver to operate a steering wheel when a steering angle is given to a steered wheel (which is normally a front wheel except in a special vehicle such as a forklift), a power steering apparatus is widely used. Also, as one type of such power steering apparatuses, an electric power steering apparatus employing an electric motor as its auxiliary power source has become widely in use. Such an electric power steering apparatus can be made in a small size with a light weight, compared with a hydraulic power steering apparatus, since the magnitude of the auxiliary power (torque) can be easily controlled with less loss of the engine power in this electric power steering apparatus.
In the case of such conventional electric power steering apparatus as described above, since a geared transmission, such as a worm transmission, is used as a speed reducing device to be disposed between an electric motor and a steering shaft, a noise, vibration, and the like are generated when the auxiliary power is given, which gives the occupants including the driver an undiscomfort ablesentation in some cases. Also, since there exists a backlash which is avoidable in the geared transmission, a response may be somewhat deteriorated to bring about poor operability in some cases. Especially, since the backlash generated in a speed reducing device portion may become a play (shake) in a direction of rotation of the steering shaft in an enlarged condition in some cases, the above-mentioned response is easily deteriorated conspicuously. These problems can be removed to some extent by reducing the backlash, or forming a gear for constituting the speed reducing device of synthetic resin. However, other problems are generated such as the control of the dimensional accuracy becoming more difficult. Since the gear made of synthetic resin has a smaller allowable surface pressure and a smaller allowable stress than those of a metallic gear made of, for example, steel, it is difficult to combine such gear with an electric motor having a large output. Moreover, since the heat-resisting performance of the gear of synthetic resin is inferior to that of a metallic gear, it is difficult to install such gear of synthetic resin in an engine compartment which rises to a high temperature (especially an engine compartment of a high performance car with a high-output engine mounted thereon). For these reasons, under present conditions, the electric power steering apparatus is used limitedly in a comparatively inexpensive small-sized car, partly also because of an output of the electric motor thereof, and a conventional hydraulic steering apparatus is used in a large-sized or expensive car.
If a frictional transmission is used as such transmission, it is possible to prevent a play which causes the above-mentioned problems. However, there still remain problems to be solved for realizing a small-sized electric power steering apparatus with a satisfactory efficiency. For example, in order to secure the energy efficiency, it is preferable to use an electric motor with a small torque, in exchange for a high rotating speed. On the other hand, when the electric motor with a small torque in exchange for a high rotating speed is used, it is required to increase a speed reducing ratio of transmission. However, in practice, according to the prior art, a small-sized frictional transmission with a speed reducing ratio and excellent transmitting efficiency has not yet been attained and a small-sized electric power steering apparatus with a high energy efficiency and an excellent response has not yet been realized. If plural frictional transmissions are arranged in series with respect to the direction of power transmission, a large speed reducing ratio can be attained. However, the structure of such arrangement is complicated to increase the cost, so that it becomes difficult to reduce the size and the weight of the electric power steering apparatus.